The present invention relates to an OFDM transmission system, an OFDM receiver, OFDM modems including ADSL modems and VDSL modems, and methods of synchronising an OFDM receiver with an incoming multi-carrier signal, in particular, the present invention relates to sampling clock oscillator control for an OFDM system.
In this specification the term OFDM (Orthogonal Frequency Division Multiplex) type is intended to include DMT (Discrete Multi-Tone).
The demand for provision of multi-media and other broad bandwidth services over telecommunications networks has created a need to transmit high bit rate traffic over copper pairs. This requirement has led to the development of a number of different transmission schemes, such as, ADSL (Asynchronous Digital Subscriber Line) and VDSL (Very high bit-rate Digital Subscriber Lines). One of the more likely modulation systems for all these transmission schemes is a line code known as DMT (discrete multi-tone), which bears a strong resemblance to orthogonal frequency division multiplex, and is a spread spectrum transmission technique.
In discrete multi-tone transmission, the available bandwidth is divided into a plurality of sub-channels each with a small bandwidth, 4 kHz perhaps. Traffic is allocated to the different sub-channels in dependence on noise power and transmission loss in each sub-channel. Each channel carries multi-level pulses capable of representing up to 11 data bits. Poor quality channels carry fewer bits, or may be completely shut down.
Because inter pair interference in copper pair cables is higher where data is transmitted in both directions, i.e. symmetric duplex, a number of transmission schemes have proposed the use of asymmetric schemes in which high data rates are transmitted in one direction only. Such schemes meet many of the demands for high bandwidth services, such as, video-on-demand.
VDSL technology resembles ADSL to a large degree, although ADSL must cater for much larger dynamic ranges and is considerably more complex as a result. VDSL is lower in cost and lower in power, and premises VDSL units need to implement a physical layer media access control for multiplexing upstream data.
Four line codes have been proposed for VDSL:
CAP; Carrierless AM/PM, a version of suppressed carrier QAM, for passive NT configurations, CAP would use QPSK upstream and a type of TDMA for multiplexing (although CAP does not preclude an FDM approach to upstream multiplexing);
DMT; Discrete Multi-Tone, a multi-carrier system using Discrete Fourier Transforms to create and demodulate individual carriers, for passive NT configurations; DMT would use FDM for upstream multiplexing (although DMT does not preclude a TDMA multiplexing strategy);
DWMT; Discrete Wavelet Multi-Tone, a multi-carrier system using Wavelet Transforms to create and demodulate individual carriers, DWMT also uses FDM for upstream multiplexing, but also allows TDMA; and
SLC; Simple Line Code, a version of four-level baseband signalling that filters the base band and restores it at the receiver, for passive NT configurations; SLC would most likely use TDMA for upstream multiplexing, although FDM is possible.
Early versions of VDSL will use frequency division multiplexing to separate downstream from upstream channels and both of them from POTS and ISDN. Echo cancellation may be required for later generation systems featuring symmetric data rates. A rather substantial distance, in frequency, will be maintained between the lowest data channel and POTS to enable very simple and cost effective POTS splitters. Normal practice would locate the downstream channel above the upstream channel. However, the DAVIC specification reverses this order to enable premises distribution of VDSL signals over coaxial cable systems.
In a multi-carrier system, such as a DMT system, a receiver must be able to recover a sampling clock that is very accurately synchronized to a transmitter sampling clock. A known method, for achieving synchronization, uses a reserved carrier, the pilot carrier, which is transmitted with a fixed phase. The receiver sampling clock is then phase locked to the pilot carrier. Frame timing must also be recovered. In existing systems this has been achieved by using a correlation technique operating in the time domain.
With OFDM systems the frequency domain data is the Fourier transform of the received time domain OFDM frames. The time domain frames must be sampled, at the receiver, in synchronism with the transmitter, so that each received frame contains data from only a single transmitted frame. It is vital for this synchronism to be maintained in order to maintain the orthogonality of the frames.
A typical multi-carrier system, of the OFDM type, which uses a cyclic prefix, permits orthogonality to be maintained when there is a small deviation from exact frame synchronisation. Because the signalling interval includes both an entire frame and the cyclic prefix, which is a repetition of part of the frame, a frame sampled within the signalling interval will contain data from only one frame. Since the signalling interval is greater than the frame period, this gives some leeway in frame alignment.
In a multi-carrier system of the OFDM type, the control of the receiver sampling clock is critical for achieving an optimal utilization of the channel capacity. The present invention takes the data needed for this control function from the received signal in a novel manner, in particular, the present invention uses adaptive equalizer parameters for sampling clock oscillator control.
In a multi-carrier system of the OFDM type, an adaptive channel equalizer, operating in the frequency domain, is often used. The internal parameters in such an equalizer contain, in addition to information about the characteristics of the channel, information which can be interpreted as the time deviation between the sampling clocks of the transmitter and the receiver. The present invention utilizes this information to control the sampling clock of the receiver in a more robust way than has previously been possible with known techniques.
Known techniques for achieving frame synchronisation do not operate entirely in the frequency domain. A technique for frame synchronisation in which only frequency domain data is employed, is described in our co-pending patent application Kgp 74/97.
The present invention is particularly suitable for use in ADSL and VDSL modems which can be used to give broadband access over copper networks with relatively stationary channels. The invention is, however, of general application and also of relevance to broadband transmission in mobile and semi-mobile systems for transmission over the radio channels.
The present invention provides an extremely robust estimation of the time deviation between the sampling clocks of the transmitter and the receiver and can handle deviations of several periods, which implies that symbol limits are also guided to the right location. The robustness is achieved by using all active carriers in the estimation.
According to a first aspect of the present invention, there is provided a receiver, for use in an OFDM type transmission system, in which data is transmitted in frames, each frame having a cyclic prefix which is a repetition of part of said frame, characterised in that control means are provided which control a sampling oscillator, and in that said control means include estimation means for estimating timing deviations of said sampling clock, said estimation means operating entirely on frequency domain input data.
According to a second aspect of the present invention, there is provided a receiver, for use in an OFDM type transmission system, in which data is transmitted in frames, each frame having a cyclic prefix which is a repetition of part of said frame, and in which said receiver has an adaptive equaliser having an equaliser inverse channel model, characterised in that separation means are provided for separating said equaliser inverse channel model into a first and a second part, said first part being independent of sample timing and said second part being dependent on sample timing and in that control means are provided which control a sampling oscillator in dependence on said second part.
Said control means may include estimation means for estimating timing deviations of said sampling clock, said estimation means operating entirely on frequency domain input data.
Said estimation means may estimate an approximation of a linear portion of an argument function produced by timing deviations of said sampling oscillator.
Said estimation means may be adapted to find a linear part of said argument function by taking an average slope of said argument function.
Said approximation of a linear portion of an argument function may be used as a feedback control signal for said sampling clock.
Said approximation of a linear portion of an argument function may have a slope which converges to zero as a control loop, for said sampling clock, settles.
Those parts of said equaliser inverse channel model, other than said linear portion of said argument function, may be controlled by said equaliser, which continuously adapts to variations in sampling timing.
Said equaliser and said control means may each use well defined and different portions of said equaliser inverse channel model to achieve an output frequency domain signal with zero phase deviation relative to a transmitted signal.
Said slope of said argument function, as, may be estimated from the equation       α    k    =            1      N        ⁢                  ∑        n            ⁢              L        ⁢                              X                          n              ,              k                                n                    
where N is the number of active carriers and Xn,k is the unwrapped argument function for the nth carrier in the kth frame.
Said slope of said argument function, xcex1k, may be estimated from the equation       α    k    =            2                        n          2                -                  n          0                      ⁡          [                                    ∑                          n              =                                                n                  1                                +                1                                                    n              2                                ⁢                      LX                          n              ,              k                                      -                              ∑                          n              =                              n                0                                                    n              1                                ⁢                      LX                          n              ,              k                                          ]      
where N is the number of active carriers, Xn,k is the unwrapped argument function for the nth active carrier in the kth frame, indices n0 and n2 are the lower and upper limits respectively of the band and index n1 divides the band into two equal parts.
On start up, frame timing may be adjusted until received frames are sampled inside a signal interval.
Means may be provided, responsive to a feed back control for said sampling oscillator, to adjust said frame timing so that frame synchronization is maintained.
According to a third aspect of the present invention, there is an OFDM type transmission system in which data is transmitted in frames, each frame having a cyclic prefix which is a repetition of part of said frame, characterised in that said system includes a receiver as defined in any preceding paragraph.
According to a fourth aspect of the present invention, there is provided, in an OFDM type system in which data is transmitted in frames, each frame having a cyclic prefix which is a repetition of part of said frame, a method of synchronising a receiver sampling oscillator with a transmitter sampling oscillator, characterised by controlling said sampling oscillator with a feedback signal representing an estimation of timing deviations of said sampling clock, said estimation signal derived directly from domain input data.
According to a fifth aspect of the present invention, there is provided, an OFDM type system in which data is transmitted in frames, each frame having a cyclic prefix which is a repetition of part of said frame, and in which said receiver has an adaptive equaliser having an equaliser inverse channel model, a method of synchronising a receiver sampling oscillator with a transmitter sampling oscillator, characterised by separating said equaliser inverse channel model into a first and a second part, said first part being independent of sample timing and said second part being dependent on sample timing and controlling a sampling oscillator in dependence on said second part.
Timing deviations of said sampling clock may be estimated entirely from frequency domain input data.
An approximation of a linear portion of an argument function produced by timing deviations of said sampling oscillator may be estimated.
A linear part of said argument function may be found by taking an average slope of said argument function.
Said approximation of a linear portion of an argument function may be used as a feedback control signal for said sampling clock.
Said approximation of a linear portion of an argument function may have a slope which converges to zero as a control loop, for said sampling clock, settles.
Those parts of said equaliser inverse channel model, other than said linear portion of said argument function, may be controlled with said equaliser, which continuously adapts to variations in sampling timing.
Said equaliser and said control means may each use well defined and different portions of said equaliser inverse channel model to achieve an output frequency domain signal with zero phase deviation relative to a transmitted signal.
Said slope of said argument function, xcex1k, may be estimated from the equation       α    k    =            1      N        ⁢                  ∑        n            ⁢              L        ⁢                              X                          n              ,              k                                n                    
where N is the number of active carriers and Xn,k is the unwrapped argument function for the nth carrier in the kth frame.
Said slope of said argument function, xcex1k, may be estimated from the equation       α    k    =            2                        n          2                -                  n          0                      ⁡          [                                    ∑                          n              =                                                n                  1                                +                1                                                    n              2                                ⁢                      LX                          n              ,              k                                      -                              ∑                          n              =                              n                0                                                    n              1                                ⁢                      LX                          n              ,              k                                          ]      
where N is the number of active carriers, Xn,k is the unwrapped argument function for the nth active carrier in the kth frame, indices n0 and n2 are the lower and upper limits respectively of the band and index n1 divides the band into two equal parts.
Frame timing, on start up, may be adjusted until received frames are sampled inside a signal interval.
Said frame timing may be adjusted in accordance with a feed back signal for said sampling oscillator, so that frame synchronization is maintained.
According to a sixth fifth aspect of the present invention, there is provided an ADSL modem characterised in that said modem has a receiver as defined above, or operates a method of synchronisation as defined above.
According to a sixth aspect ofthe present invention, there is provided a VDSL modem characterised in that said modem has a receiver as defined above, or operates a method of synchronisation as defined above.